1. Field of the Invention
The present invention relates to an optical beam scanner which is incorporated in an optical system of a laser printer, and more particular, the present invention, it relates to a control system of a diaphragm plate therein.
2. Description of the Related Art
There has been a need for the realization of a compact scanner which can be incorporated in an optical system of a laser printer, etc., in accordance with the miniaturization of the laser printer. In a known reflection-type compact beam scanner, the laser beam deflected by a polygonal mirror is reflected by a curved mirror in the sub-scanning direction at a predetermined angle toward the polygonal to mirror laser beam is made incident upon an object surface to be scanned through an anamorphic lens, as disclosed in Japanese Unexamined Patent Publication No. 6-265810 (referred to as "JPP '810" hereinafter) for example.
The reflection surfaces of the polygonal mirror are parallel with the center axis (rotation axis) of the polygonal mirror about which the polygonal mirror rotates. Namely, assuming that a reference plane is defined by a plane perpendicular to the center axis of the polygonal mirror, the reflection surfaces are perpendicular to the reference plane. Since a bundle of light (light bundle) emitted from the light source is separated in the sub-scanning direction by the polygonal mirror, the light source is disposed such that the light incident upon the polygonal mirror defines a predetermined angle with respect to the reference plane in the sub-scanning plane.
However, in the known reflection-type beam scanner mentioned above, the light path from the light source to the polygonal mirror is defined by a straight line since no reflection system is used. Therefore, it is impossible to make the length of the beam scanner in the direction of the straight light path smaller than the distance between the light source and the polygonal mirror. Hence, the beam scanner cannot be miniaturized.
A solution to miniaturize an optical system is to bend the light path using a mirror disposed in the light path from the light source to the polygonal mirror. However, this solution is not useful for miniaturizing and the optical system in the main scanning direction in which the light bundle is moved to scan an object, since the size of the optical system in the main scanning direction is determined by the width of the curve mirror and the anamorphic lens. Namely, even if the light path from the light source to the polygonal mirror is bent into the main scanning direction, substantial reduction in the size of the entire optical system cannot be expected.
In the simplest arrangement to bend the light path using a plane mirror, the latter is added in the optical system as disclosed in JPP '810 mentioned above, so that the positional relationship between the light source and the polygonal mirror is optically equivalent to that in the optical system in JPP '810. The line normal to the plane mirror is parallel with the reference plane. Hence, the light path which is developed without taking the reflection by the mirror into consideration is identical to that in JPP '810. In this arrangement, the space for the optical system in the direction of travel of the light incident upon the polygonal mirror is reduced, nevertheless, the height of the optical system in the sub-scanning direction cannot be decreased in comparison with the prior art.
If the direction of the light incident upon the plane mirror is made parallel with the reference plane, the height of the optical system in the sub-scanning direction can be decreased. In order to achieve this, it is necessary to incline the line normal to the plane mirror with respect to the reference plane, instead of inclining the light incident upon the polygonal mirror with respect to the reference plane.
In a first arrangement in which the normal line to the plane mirror is inclined with respect to the reference surface, the reference plane itself is inclined with respect to the bottom surface of the casing; the light path of light incident upon the plane mirror and the normal line to the plane mirror are both parallel with the bottom surface of the casing; and the center axis (rotation axis) of the polygonal mirror is inclined with respect to the bottom surface of the casing. In the first arrangement, the mounting to the casing and the adjusting of the polygonal mirror are difficult.
In a second arrangement in which the normal line to the plane mirror is inclined with respect to the reference surface, the reference plane is parallel with the bottom surface of the casing; the light path of light incident upon the plane mirror is parallel with the bottom surface of the casing; the center axis (rotation axis) of the polygonal mirror is perpendicular to the bottom surface of the casing; and the normal line to the plane mirror is inclined with respect to the bottom surface of the casing. In the second arrangement, there is a problem in that a beam is not correctly focused on a surface to be scanned (i.e., the shape of the beam spot is deformed). This will be discussed below.
In general, in an optical beam scanner, it is preferable that the shape of the beam spot on the image surface be elongated in the sub-scanning direction. In order to achieve this, a diaphragm plate having a slit which as a major diameter in the main scanning direction and a minor diameter in the sub-scanning direction is provided in the vicinity of the light source. The diameter of the light bundle differs in the main scanning direction and sub-scanning direction due to the slit, so that the f-number of the optical system is substantially varied to thereby vary the diameter of the beam spot.
However, in the second arrangement in which the normal line to the plane mirror is inclined with respect to the bottom surface of the casing, the direction of the section of the light bundle is distorted upon reflection by the plane mirror. Consequently, if the longitudinal direction of the slit is parallel with the bottom surface of the casing as in the conventional scanner, the direction corresponding to the longitudinal direction of the slit does not coincide with the main scanning direction on the reflection surface of the polygonal mirror due to the distortion. Therefore, the direction in which the f-number is substantially reduced is not identical to the main scanning direction. Hence, the direction of the beam spot formed on the surface to be scanned is rotated with respect to an ideal direction by a predetermined angle. If an ideal beam spot having a minor diameter in the main scanning direction is rotated, the spot diameter in the main scanning direction, i.e., the size of the beam spot projected on the line in the main scanning direction is increased, so that the resolution of the image is attenuated. Consequently, the required performance of the beam scanner cannot be achieved.